[release-branch.go1.5] runtime: ignore rt_sigaction error if it is for SIGRTMAX

A forward port of https://codereview.appspot.com/124900043/ which somehow
got lost somewhere.

Fixes #13024

Change-Id: Iab128899e65c51d90f6704e3e1b2fc9326e3a1c2
Reviewed-on: https://go-review.googlesource.com/16853


Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/16986


Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>